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For alternate meanings see Star (disambiguation)

Hundreds of stars are visible in this image of the Sagittarius Star Cloud in our Milky Way Galaxy taken by the Hubble Space Telescope.

A star is a large glowing sphere that is found in outer space. Stars appear as points in the nighttime sky that twinkle because of the effect of the Earth's atmosphere. The Sun is an exception: it is the only star sufficiently close to Earth to appear as a disc and to provide daylight.

Common language does not always reflect this astronomical usage; the term "star" ordinarily does not include the Sun, and sometimes includes the visible planets and even meteors ("shooting stars" or "falling stars").

The nearest star to the earth, apart from the Sun, is Proxima Centauri, which is 40 trillion kilometers away. Light from Proxima Centauri takes 4.2 years to reach Earth. (See light year.) If you took the French TGV, one of the fastest trains, on a trip to Proxima Centauri using its highest recorded speed (515.3 kilometers per hour), it would take you about 8.86 million years.

Astronomers estimate that there are at least 70 sextillion stars in the known universe (7 x 1022). That is 70 000 000 000 000 000 000 000.

Many stars are between 1 billion and 10 billion years old. Some stars may even be close to 13.7 billion years old, which is the estimated age of the universe. (See Big Bang theory and Stellar evolution.) They range in size from the tiny neutron stars (which are actually dead stars) no bigger than a city, to supergiants like the North Star (Polaris) and Betelgeuse, in the Orion constellation, which have a diameter about 1,000 times larger than the sun—about 1.6 billion kilometers.

Scientifically, stars are defined as self-gravitating spheres of plasma in hydrostatic equilibrium, which generate their own energy through the process of nuclear fusion.

The energy produced by stars radiates into space as electromagnetic radiation (mostly visible light), and as a stream of neutrinos. The apparent brightness of a star is measured by its apparent magnitude.

Stellar astronomy is the study of stars and the phenomena exhibited by the various forms/developmental stages of stars.

Many stars are gravitationally bound to other stars, forming binary stars. Larger groups called star clusters also exist. Stars are not spread uniformly across the universe, but are typically grouped into galaxies. A typical galaxy contains hundreds of billions of stars.

Table of contents
1 Star formation and evolution
2 Star classification
3 Naming of stars
4 Nuclear fusion reaction pathways
5 Related topics

Star formation and evolution

As learned by star formation astronomers, stars are born in molecular clouds, large regions of slightly higher density of matter (though still less dense than the inside of an earthly vacuum chamber), and form by gravitational instability inside those clouds triggered by shockwaves from supernovae. (High mass stars powerfully illuminate the clouds from which they formed. One example of such reflection nebulae is the Orion Nebula.)

Stars spend about 90% of their lifetime fusing hydrogen to produce helium in high pressure reactions near the core. Such stars are said to be on the main sequence.

Small stars (called red dwarfs) burn their fuel very slowly and last tens to hundreds of billions of years (far longer than the time elapsed in the universe so far). At the end of their lives, they simply become dimmer and dimmer, fading into black dwarfs.

As most stars exhaust their supply of hydrogen, their outer layers expand and cool to form a red giant. (In about 5 billion years, when the sun is a red giant, it will subsume Mercury and Venus.) Eventually the core is compressed enough to start helium fusion, and the star heats up and contracts. (Larger stars will also fuse heavier elements, all the way to iron.)

An average-size star will then go nova, shedding its outer layers as a planetary nebula. The core that remains will be a tiny ball of degenerate matter not massive enough for further fusion to take place, supported only by degeneracy pressure, called a white dwarf. It will fade into a black dwarf over absurdly long stretches of time.

In larger stars, fusion continues until collapse ends up causing the star to explode in a supernova. This is the only cosmic process that happens on human timescales; historically, supernovae have been observed as "new stars" where none existed before. Most of the matter in a star is blown away in the explosion (forming nebulae such as the Crab Nebula) but what remains will collapse into a neutron star (a pulsar or X-ray burster) or, in the case of the largest stars, a black hole.

The blown-off outer layers includes heavy elements, which are often converted into new stars and/or planets. The outflow from supernovae and the stellar wind of large stars play an important part in shaping the interstellar medium.

Stellar evolution explains how stars are created and die in greater detail.

Star classification

There are different classifications of stars ranging from type O which are very large and bright, to M which is often just large enough to start ignition of the hydrogen. Some of the more common classifications are O,B,A,F,G,K,M, and can perhaps be more easily remembered using the mnemonic "Oh Be A Fine Girl, Kiss Me", invented by Annie Jump Cannon (1863-1941). There are many other mnemonics for star classification. Each letter has 9 subclassifications. Our sun is a G2, which is very near the middle in terms of quantities observed. Most stars fall into the main sequence which is a description of stars based on their absolute magnitude and spectral type.

The sun is taken as the prototypical star (not because it is special in any way, but because is the closest and most studied star we have), and most characteristics of other stars are usually given in solar units.
For example, the mass of the sun is

Msun = 1.9891 1030 kg

The masses of all other stars are given in terms of Msun.

Naming of stars

Most stars are identified only by catalog numbers; only a few have names as such. The names are either traditional names (mostly from Arabic), Flamsteed designations or Bayer designations. The only body which has been recognized by the scientific community as having competence to name stars or other celestial bodies is the International Astronomical Union. A number of private companies (e.g. the "International Star Registry") purport to sell names to stars; however, these names are not recognized by the scientific community, nor used by them. (Many in the astronomy community view these organizations as frauds preying on people ignorant of how stars are in fact named.) See star designations for more information on how stars are named.

Nuclear fusion reaction pathways

A variety of different nuclear fusion reactions take place inside the cores of stars, depending upon their mass and composition (see Stellar nucleosynthesis).

Stars begin as a cloud of mostly hydrogen with about 25% helium and heavier elements in smaller quantities. In the Sun, with a 107 K core, hydrogen fuses to form helium in the proton-proton chain:

2(1H + 1H → 2D + e- + νe) (4.0 MeV + 1.0 MeV)
2(1H + 2D → 3He + &gamma) (5.5 MeV)
3He + 3He → 4He + 1H + 1H (12.9 MeV)

These reactions result in the overall reaction:

41H → 4He + 2e- + 2γ + 2νe (26.7 MeV)

In more massive stars, helium is produced in a cycle of reactions catalyzed by carbon, the carbon-nitrogen-oxygen cycle.

In stars with cores at 108 K and masses between 0.5 and 10 solar masses, helium can be transformed into carbon in the triple-alpha process:

4He + 4He + 92 keV → 8*Be
4He + 8*Be + 67 keV → 12*C
12*C → 12C + γ + 7.4 MeV

For an overall reaction of:

34He → 12C + γ + 7.2 MeV

Related topics

See also: Blue straggler